Mercaptopurine (6-MP or 6-thiopurine) and azathioprine [6-(1-methyl-4-nitro-5-imidazolylthio) purine] are cytotoxic drugs that are effective in the treatment of ulcerative colitis and Crohn's disease (see, Present et al., Annals of Internal Medicine 111:641-649 (1989)). The prodrug azathioprine (AZA) is rapidly converted to 6-mercaptopurine through non-enzymatic, nucleophilic attack by sulfhydryl-containing compounds in the circulation. 6-MP and AZA, which are forms of the same drug and metabolic precursors of the active components, are acted upon by at least three competing enzymatic pathways. As shown in FIG. 1, several major enzyme pathways are involved. Xanthine oxidase (XO) converts 6-mercaptopurine to 6-thiouric acid. Hypoxanthine phosphoribosyl transferase (HPRT) converts 6-mercaptopurine to 6-thioinosine-5′-monophosphate, which is a precursor to 6-thioguanine nucleotides. Thiopurine methyltransferase (TPMT) catalyzes the S-methylation of 6-mercaptopurine to methylmercaptopurine (6-MMP). Thus, 6-mercaptopurine is enzymatically converted to various metabolites, including 6-thioguanine (6-TG) and 6-thioguanine nucleotides, which are the presumptive active metabolites mediating the effects of azathioprine/6-mercaptopurine drug therapy.
The interplay of the pathways described above is genetically determined and creates a highly individualized response to azathioprine/6-mercaptopurine drug therapy. The population frequency distribution of TPMT enzyme is trimodal, with the majority of individuals (89%) having high activity, 11% having intermediate activity, and about 1 in 300 (0.33%) having undetectable activity (see, Weinshilboum and Sladek, Amer. J. Human Genetics 32:651-662 (1980)). Such a trimodal relationship has been confirmed by direct measurements of TPMT enzyme activity by the Kroplin HPLC assay method (see, Kroplin et al., Eur. J. Clin. Pharmacol., 54 265-271 (1998)). In contrast to variation in TPMT activity, there is very little inter-individual variation in XO activity and only limited data on HPRT activity (see, Lennard, Eur. J. Clin. Pharm., 43:329-339 (1992)).
In certain populations, very high levels of methylated metabolites (e.g., 6-methyl-mercaptopurine (6-MMP)) are seen in red blood cells with normal thiopurine methyltransferase (TPMT) activity. This phenomenon has gone unexplained. There is contradictory evidence in the literature that high levels of methylated metabolites are associated with hepatotoxicity. Interestingly, when these patients are treated with a combination of allopurinol and azathioprine, methylated metabolites return to normal and thioguanine nucleotide levels can be pushed into the therapeutic range.
In view of the foregoing, there is a need in the art to understand the genetic interplay of the pathways described above as to create a highly individualized dose of a 6-mercaptopurine producing drug. The present invention satisfies this and other needs.